The present invention relates to a chopper type switching power supply for boosting input voltage and, more particularly, to a chopper type switching power supply in which an oscillation circuit for controlling the on/off operation of a switching element is formed by an oscillation circuit which performs the oscillating operation by using a pair of inductors coupled to each other.
As one of the chopper type switching power supplies capable of obtaining boosted DC output by using a simple circuit arrangement, there is a self-excited chopper type switching power supply in which a switching element forms a part of an oscillation circuit, as shown in the block 2 of FIG. 2 of JP-A-62-48259. However, when obtaining the DC output exceeding 10 W, for example, by using the self-excited chopper type switching power supply, such a problem is apt to occur that the oscillating operation stops when the load changes. In contrast, when the constants of elements are set so that the oscillating operation stops scarcely, the conversion efficiency is likely reduced. Thus, when obtaining the DC output of relatively large electric power such as 10 W etc., a separately excited type arrangement is employed in which an oscillation circuit is provided separately from the switching circuit and the on/off operation of the switching element is controlled by the output of the oscillation circuit.
FIG. 4 shows the aforesaid chopper type switching power supply. That is, the chopper type switching power supply is provided with a triangular wave generation circuit 72 for generating a rectangular waveform signal and a comparator 71 having one terminal to which the rectangular waveform signal generated by the triangular wave generation circuit 72 is applied and the other terminal to which divided voltage of a DC output 73 is applied. Further, the chopper type switching power supply is arranged to perform the on/off control of a transistor 74 serving as a switching element by using the output of the comparator 71.
However, the following problem arises when the aforesaid arrangement is employed. That is, the chopper type switching power supply changes a ratio between a time period where the transistor 74 is in an ON state and a time period where the transistor is in an OFF state thereby to control the DC output 73 to a predetermined voltage. Further, the period where the transistor 74 is turned on is same as the period of the triangular waveform signal generated by the triangular wave generation circuit 72. Thus, the smaller the load is, the smaller the ratio of the time where the transistor 74 is in an ON state becomes. Accordingly, when the load becomes almost 0, the time period where the transistor 74 is in an ON state becomes quite short. As a result, there arises a phenomenon that the transistor 74 is turned off on the way where the transistor moves to the saturation state of ON state. Thus, there arises a problem that, when the load becomes smaller, the loss of the transistor 74 increases and hence the conversion efficiency is reduced.